All across the United States, there are hundreds of CrossFit classes for kids. We went to several gyms in the Southland, including CrossFit LA in Santa Monica.

“We run, we jump, we push, we pull,” Brown says.

Dan Wells is the owner and coach of CrossFit Horsepower in Studio City, and while he says the kids’ classes are essentially versions of the adult classes, including weights, it is not too extreme or too dangerous for their age.

“It’s a high repetition of a weight that they can handle, which starts to get them the cardio side of it,” Wells said. “It’s functional movements, so they learn how to use their body as efficiently as possible.”

But with weights and kettle bells, is this too hard on children? Should kids be lifting more than a jump rope or kicking a soccer ball at this age?

“When I first heard about CrossFit, and I first tried it, I’m thinking ‘this exercising is a lot of work,’ but it’s really fun,” one girl told CBS2’s Lisa Sigell.

The American Academy of Pediatrics says according to research, it’s safe for kids to start light weightlifting after the age of eight. However, some of these kids are just five years old.

Dr. Tracy Zaslow, who specializes in pediatric sports medicine at Children’s Hospital Los Angeles, says there is a danger, because as kids grow, their bones, muscles and tendons are maturing.

“Specifically the growth plates, where growth occurs at the bone, is their weak link, and so it’s the most vulnerable area to injury,” Zaslow says.

Dr. Zaslow also says kids may have expectations of becoming “mini-Schwarzeneggers.” Muscle change won’t happen until puberty, but because kids are tested on their progress, they may test their bodies too hard.

“That’s the concern of doing activities like CrossFit. It’s that they will push themselves more than they would in a free-play setting,” Zaslow says.

“We’re not loading the kids with weight, we’re not pushing them to go past their limits,” Brown says. “We want them to have fun, be engaged, and move their bodies.”

Wells also says that part of the class is teaching kids when they need to stop, and the developers of CrossFit say significant injury has not been a reported through any of their programs.

“We have not had a single report of a significant injury suffered by a child. That’s hundreds of programs worldwide.”